With the advancing usage of wirelessly controlled application networks, such as for instance—but not limited to—mesh networks according to the ZigBee specification, in particular professional wireless lighting control applications, reliable transmission of control commands from a source node to a destination node constitutes an important aspect for a corresponding application. For instance, a command to switch on or off a main light or a series of lights in an important event in a big ballroom, a luxury hotel or a television show may be crucial in order to achieve the desired show effect. A corresponding control command is delivered from a central control node to a wireless receiving node inside a lighting device via many intermediate nodes, also referred to as forwarding nodes. Especially in a large network, routes between a source node and a receiving node may be very long. The success rate of a wireless transmission through a long path depends on many factors, such as the RF data rate, RF interferences, channel conditions, processing power of individual nodes. Especially in a large and dense wireless network with many nodes, the reliability of individual nodes is very poor.
It is known from “Neighbor Table Based Shortcut Tree Routing in ZigBee Wireless Networks”, by Taehong Kim, et al., published in IEEE Transactions on Parallel and Distributed Systems, vol 25, No. 3, March 2014 that different tree routing schemes are known for ZigBee, wherein this paper introduces Shortcut Tree Routing. The main idea of the shortcut tree routing being to calculate remaining hops from an arbitrary source to the destination using the hierarchical addressing scheme in ZigBee and then having each source or intermediate node forward a packet to the neighbor node with the smallest remaining hops in its neighbor table.
It is further known from “Reliable Broadcast in ZigBee Networks”, by Gang Ding, et al., published at Sensor and ad hoc Communications and Networks, 2005 that broadcasts in ZigBee networks are not very efficient; in line therewith a pruning broadcast algorithm is suggested that performs a forward node selection algorithm, this algorithm involves the selection of 1-hop neighbors to cover 2-hop neighbors that are known without exchanging information.
However, the influence of individual nodes is especially important for so called unicast transmissions where a sending node addresses a single destination node. In order to ensure proper transmission of a data message from one node to another, many network specifications, inter alia the ZigBee specification, use resubmission, acknowledgement feedback or combinations thereof. In the ZigBee specification, the reliability of a unicast transmission is handled by two layers. Every node that receives a data message provides a corresponding acknowledgement to be signaled to the sending nodes as feedback. When there is any interruption of a transmission along the delivering path, e.g. along one of a plurality of intermediate nodes, a data packet for which no acknowledgement has been received by the sending nodes will be transmit again until it reaches the next node. This retransmission based upon a missing acknowledgement, improves the reliability of a transmission. On the other hand, the time for delivering a data messages increases as well, since the retry mechanism is based on timeout: if a sending node fails to receive acknowledgement from the next node after a predetermined time interval has lapsed, the sending node will resend the message to the next node. All intermediate nodes along the delivering path use that mechanism resulting in a unicast delivering time that is highly variable depending on the delivering conditions of each individual node. The second layer of the reliability mechanism is the response feedback from the destination node of a unicast transmission. Upon receipt of the transmission from the source node the destination node will send back a packet or response to the source node, using the reversed delivering path from the destination node to the source node. If the source node has not received the response from the destination node after a given period of time, the source node will try to resend the message. This layer requires an even longer waiting time, since each node may need to issue a retry as explained herein above. If the delivering route is long, the uncertainty on the accumulated time intervals may be such that a delivering time is not predictable. Hence, the current ZigBee unicast algorithm performances well in simple networks with low external RF interference, but performance drops significantly in larger networks or in networks with heavy external RF interferences, e.g. from wireless mobile devices using different RF technologies. Simply introducing redundant concurrent delivering paths would increase the reliability of a proper data transmission. However, such an approach would have a negative impact on the unicast performance. It would reduce the delivering speed, and allocate more channel capacity.